1. Field of the Invention
The present invention relates to a transmission device and a transmission method used in digital communication, and more particularly to a transmission device and a transmission method capable of detecting the transmission rate of a received digital signal.
2. Description of the Related Art
In a conventional communication system, a signal at a predetermined transmission rate is usually transmitted through each transmission line. However, as a photonic network to which wavelength division multiplexing (WDM) transmission is applied and in which optical crossconnects and optical add/drop multiplexers (ADM) are used has been in use in recent years, a plurality of optical signals have become transmitted through WDM technology. In this case, it is more likely that a plurality of optical signals, each at its own transmission rate, travel in the same transmission line. In such a network, when receiving each optical signal demultiplexed from the wavelength-multiplexed signals via the ADM and so on, the transmission rate of the signal must be identified.
As an example of a circuit identifying the transmission rate, a serial-data transmission rate detection circuit is disclosed in Japanese Patent Laid-Open Publication No. Hei 2-11049. This circuit counts the number of sampling clock pulses at least two times higher than the maximum rate of the expected transmission data for the duration equivalent to one bit of received serial data. The circuit detects the length of one-bit interval based on the count result and detects the rate of the transmission data. The circuit in the prior art described above requires a circuit operating at least two times the data transmission rate.
Recently, the Synchronous Transfer Mode (STM)-N is becoming the worldwide standard for the trunk line optical transmission system. The transmission rate of STM-N is based on 155.52 Mb/s that is the transmission rate of STM-1. A bit rate higher than STM-1 is set to an N multiples of 155.52 Mb/s (where, N is an integer). Out of these rates, the Consultative Committee on International Telegraphy and Telephony (CCITT), a permanent group of the International Telecommunication Union (ITU), recommends two rates as the standards: 622.08 Mb/s corresponding to N=4 and 2488.32 Mb/s corresponding to N=16. When N=64, the transmission rate is 9953.28 Mb/s. In an actual transmission system, these transfer modes are sometimes mixed. In such a case, the receiving side must check the mode in which data is transmitted in order to process received signals according to each transmission mode. However, the prior art described above requires a circuit operating twice as high as the data transmission rate. In STM-64, the circuit operation speed is as high as 20 Gb/s or higher. Such a high-speed circuit requires sophisticated manufacturing technology, resulting in increased costs.
It is an object of the present invention to provide a transmission device and a transmission method that are simple in circuit configuration and are capable of detecting the transmission rate without having to perform complex operations such as maintaining synchronization with input digital signals.
According to one aspect of the present invention, there is provided a transmission device comprising a change-point counter counting, for a predetermined time interval, a number of change points in code included in an input digital signal for outputting a count result; and a transmission rate identification circuit determining a transmission rate of the input digital signal based on the count result.
According to another aspect of the present invention, there is provided a transmission device comprising a plurality of frequency dividers each receiving an input digital signal and each having an frequency division ratio; a plurality of change-point counters each counting, for a predetermined time interval, a number of change points in code included in each of outputs of the plurality of frequency dividers for outputting a count result; and a transmission rate identification circuit determining a transmission rate of the input digital signal based on the count result.
According to still another aspect of the present invention, there is provided a transmission method comprising the steps of counting, for a predetermined time interval, a number of change points in code included in an input digital signal for outputting a count result; and identifying a transmission rate of the input digital signal based on the count result.